The present invention relates to a process for the preparation of pulverulent, pourable elastomer-filler mixtures optionally containing plasticizer oil. The process conventionally involves mixing an aqueous emulsion of an elastomer solution with an aqueous dispersion of fillers; introducing this mixture into water containing precipitants; precipitating the elastomer-filler mixture while simultaneously removing the elastomer solvent by distillation; separating the thus-precipitated elastomer-filler mixture from the water; and drying the elastomer mixture under constant agitation.
A large number of publications have been issued and numerous patent applications have been filed over more than a decade, regarding the purposes and objectives of using pulverulent elastomers and possible methods for the production thereof, clearly indicating the great popularity of pulverulent elastomers for the modern rubber processing industry.
This ever-increasing interest in pulverulent elastomers can be readily understood by a consideration of the present state of the art of processing technology in the rubber industry. Presently, in this industry, elastomer mixtures are prepared in a discontinuous mode of operation. The principal reason for this is the physical condition of the rubber raw material, which is in bale form. The comminution of the bale and its intimate mixing with fillers, mineral oil plasticizers and auxiliary vulcanizing agents take place on rolls or in internal mixers. The operation of these units requires high amounts of energy. During the agitation of the highly viscous material, this energy is rapidly converted into heat. To avoid deterioration in quality by molecular changes or premature vulcanization, the mixture is produced in several process stages. Between these stages, the mixture is generally placed in storage. The internal mixers or rolls are followed by extruder pelletizers or extruder roller dies. This discontinuous method of processing requires a high expenditure in time, energy, and personnel.
Thus, the present state of the rubber processing art is highly unsatisfactory. This situation can only be remedied by a completely novel processing technology. Consequently, in recent years, discussions of alternative techniques have centered around the use of pourable elastomer powders since without a doubt raw materials in this physical form greatly reduce the expenditures in technology, personnel, and energy for the rubber industry. Moreover, they will provide the conditions for a one-stage and automated processing operation, while eliminating the heretofore customary heavy machinery. The development of such pourable powders creates the possibility of processing elastomer mixtures in the same way as synthetic resin powders.
At the same time, it is required that the new suitable product concept optimally meet the aforementioned requirements and overcome the above-described difficulties. In addition, there is a need for making available maximally economical manufacturing processes with the elimination of the use of undesired foreign substances. As a result, the development has perforce led to pulverulent premixes of elastomers and fillers. Because of economical considerations regarding manufacturing and processing, oil-free blends have evolved as the preferred item.
Most recently, several methods have been discovered and described for the manufacture of pulverulent, pourable elastomer-filler mixtures, preferably elastomer-carbon black mixtures based on all-purpose rubbers. See German Patent Applications P 21 35 266 (U.S. Pat. No. 3,846,365); P 22 14 121 (U.S. Pat. No. 3,895,035); P 22 60 340 (U.S. Pat. No. 3,920,604); P 23 24 009 (U.S. Pat. No. 3,945,978); P 23 25 550 (U.S. Pat. No. 3,922,240); P 23 32 796 (U.S. Pat. No. 3,929,707); and, P 24 39 237 (U.S. Pat. No. 4,073,755), whose disclosures are incorporated by reference.
Essential features of the processes described in these prior art publications comprise the utilization of specific, surface-active compounds in combination with quite specific process steps. Because of the presence of the active fillers, which are necessary anyway in the subsequent processing, surface structures are obtained which impart an excellent shelf life to the pourable powders, including favorable transportation properties. The dominant advantage, however, lies in the optimum preliminary dispersion of the fillers, offering extremely favorable conditions for the development of low-energy, continuous and fully automatic processing operations.
The conventional processes for the preparation of pulverulent elastomer mixtures comprise the following features in accordance with the cited state of the art:
1. The solutions of the above-mentioned elastomers are emulsified in water with the aid of cationic or nonionic, nitrogen-containing surfactants; mixed with aqueous dispersions or fillers, preferably carbon blacks; and the stable mixtures from the aqueous emulsions of the rubber solutions and the fillers are subjected to a one-stage precipitating process.
2. The summary of the aforedescribed processes can also be expanded to include anionic surfactants; however, then a three-stage precipitation technique must be employed while maintaining certain process characteristics (see German Patent Application No. P 24 39 237).
The heretofore described processes accordingly require either relatively expensive chemicals, especially costly surfactants such as, for example, quaternary ammonium salts or alkyl amine oxyalkylates (condensation products of alkyl amines with ethylene oxide), in relatively large quantities, or an expensive, multistage precipitation technique. Since, as has been explained above, the primary objective in the use of pulverulent elastomers is an increase of economy in the area of rubber processing, a basic prerequisite for the success of such efforts is the inexpensive production of elastomers in powder form. Thus, there remains a genuine need for an improved process for production of powdered elastomer-filler mixtures both from economic and technological viewpoints.